Field of the Invention
The present invention relates to a metal material and a method for manufacturing the same, and particularly relates to, for example, a silver material, a copper material, and an aluminum material that are used for power transmission cables, wiring materials between audio devices or electronic devices, or between components thereof, bonding wires, and the like.
Description of the Related Art
Oxygen-free copper (OFC), silver-containing and oxygen-free copper, zirconium-containing and oxygen-free copper, and the like are widely used as wiring materials for connecting electronic components constituting audio equipment or video equipment. These wiring materials are known to have relatively high electric conduction efficiency compared to commonly used copper wires, but are also known to have fine crystal structures, and thus grain boundaries, which exist in the direction in which electrons are conducted, and impurities such as sulfides and intermetallic compounds have adverse effects on the electric conduction efficiency. The reason for this seems to be that grain boundaries and impurities accumulated therein cause an increase in electrical resistance, or serve as capacitors having very small capacitance, which leads to introduction of electrostatic capacity.
To achieve an improvement in this respect, the technique disclosed in Japanese Patent Laid-Open No. S60-003808 performs coarsening of crystal grains of OFC by heat treatment and then performs wire drawing to orient the crystal grains in the longitudinal direction. This technique is to decrease the number of grain boundaries by significantly coarsening the crystal grains, but the coarsened crystal grains are unfortunately destroyed by external stress in the process of manufacture of a metal material through plastic working called wire drawing, thus disturbing the crystal structure and causing lattice defects such as lattice vacancies and dislocations. This then results in the problem that these lattice defects act to cause an increase in electrical resistance and the formation of electrostatic capacity as with impurities.
In general, metals that have been deformed by plastic working such as rolling cause working strain, producing lattice distortions, defects and the like in the crystals. The technique disclosed in Japanese Patent Laid-Open No. S63-174217, which focuses on an improvement in the manufacturing process of metal materials, is intended to solve this problem.
Japanese Patent Laid-Open No. S63-174217 discloses that a manufactured metal material has outstanding signal transmission characteristics by using, as a signal transmission copper wire, a linear-bar-shaped ingot having a single-crystal structure or a directionally solidified structure in the longitudinal direction of OFC, or the linear-bar-shaped ingot on which plastic working has been applied by slight wire drawing or the like, and providing the copper wire with an electrical conductivity of 100% IACS (International Anneld Copper Standard) or more, or a tensile strength of 20 kg/mm2 or less.
This technique was achieved by improving the problem wherein the above-described lattice defects produced during working have been a cause of deterioration in signal transmission characteristics. Meanwhile, a directionally solidified structure has a smaller number of grain boundaries, which prevent the movement of electrons, and oxygen, and hydrogen gas, and other impurities are discharged from the solidification interface into molten metal during casting, making defects less likely to be caused thereby.
Microscopically, metal materials having a polycrystalline structure inevitably experience losses during electrical conduction due to the presence of many grain boundaries therein. The technique according to Patent reference 2 achieves improved signal transmission characteristics by using a signal transmission copper wire formed from a linear-bar-shaped ingot having a single-crystal structure or a directionally solidified structure in the longitudinal direction, or the linear-bar-shaped ingot on which plastic working has been applied by slight wire drawing or the like. However, provision of linear-bar-shaped ingots having a single-crystal structure and a directionally solidified structure requires a very complicated metal solidification control process by heating casting mold continuous casting or the Czochraski method, as well as a great amount of time, resulting in the disadvantages of enormous cost and extreme difficulty in achieving mass production within a predetermined time period.
This has resulted in a critical problem in that the materials are expensive and can be obtained only in small quantities, preventing their industrial application. Additionally, this method requires an even longer time for formation of a wire rod having a large diameter, thus inevitably making the manufacture of a plate-shaped material and a strip-shaped material very difficult.